Method of fabricating nitride semiconductor device

ABSTRACT

The method of fabricating a nitride semiconductor device of this invention includes plural steps of respectively growing plural nitride semiconductor layers on a substrate; and between a step of growing one nitride semiconductor layer and a step of growing another nitride semiconductor layer adjacent to the one nitride semiconductor layer among the plural steps, a step of changing a growth ambient pressure from a first growth ambient pressure to a second growth ambient pressure different from the first growth ambient pressure.

BACKGROUND OF THE INVENTION

The present invention relates to a method of fabricating a nitridesemiconductor device such as a semiconductor laser diode expected to beapplied to the fields of optical information processing and the like.

Recently, a nitride semiconductor of a group III-V compound, that is, acompound including nitride (N) as a group V element, is regarded as apromising material for a short-wavelength light emitting device due toits large energy gap. In particular, a gallium nitride-based compoundsemiconductor (Al_(x)Ga_(y)In_(z)N, wherein 0≦x, y, z≦1 and x+y+z=1) hasbeen earnestly studied and developed, resulting in realizing a practicalblue or green light emitting diode (LED) device. Furthermore, inaccordance with capacity increase of an optical disk unit, asemiconductor laser diode lasing at a wavelength of approximately 400 nmis earnestly desired, and a semiconductor laser diode using a galliumnitride-based semiconductor is to be practically used.

Now, a conventional gallium nitride-based semiconductor laser diode willbe described with reference to a drawing.

FIG. 11 shows the sectional structure of the conventional galliumnitride-based semiconductor laser diode showing laser action. As isshown in FIG. 11, the conventional semiconductor laser diode includes abuffer layer 302 of gallium nitride (GaN), an n-type contact layer 303of n-type GaN, a first cladding layer 304 of n-type aluminum galliumnitride (AlGaN), a first light guiding layer 305 of n-type GaN, amultiple quantum well (MQW) active layer 306 including gallium indiumnitride layers having different composition ratios of indium(Ga_(1-x)In_(x)N/Ga_(1-y)In_(y)N wherein 0<y<x<1), a second lightguiding layer 307 of P-type GaN, a second cladding layer 308 of p-typeAlGaN and a p-type contact layer 309 of p-type GaN successively formedon a substrate 301 of sapphire by, for example, metal organic vaporphase epitaxial growth (MOVPE).

An upper portion of the second cladding layer 308 and the p-type contactlayer 309 are formed into a ridge with a width of approximately 3through 10 μm. A lamination body including the MQW active layer 306formed on the semiconductor substrate 301 is etched so as to expose partof the n-type contact layer 303, and the top face and the side faces ofthe etched lamination body are covered with an insulating film 310. In aportion of the insulating film 310 above the p-type contact layer 309, astripe-shaped opening is formed, and a p-side electrode 311 in ohmiccontact with the p-type contact layer 309 through the opening is formedover a portion of the insulating film 310 above the ridge. Also, on aportion of the n-type contact layer 303 not covered with the insulatingfilm 310, an n-side electrode 312 in ohmic contact with the n-typecontact layer 303 is formed.

In the semiconductor laser diode having the aforementioned structure,when a predetermined voltage is applied to the p-side electrode 311 withthe n-side electrode 312 grounded, optical gain is generated within theMQW active layer 306, so as to show laser action at a wavelength ofapproximately 400 nm. The wavelength of the laser action depends uponthe composition ratios x and y or the thicknesses of the Ga_(1-x)In_(x)Nand Ga_(1-y)In_(y)N layers included in the MQW active layer 306. Atpresent, the semiconductor laser diode having this structure has beendeveloped to show continuous laser action at room temperature or more.

It is generally well known that the growth temperature for growing anitride semiconductor crystal by the MOVPE is changed in accordance withthe composition ratio of a group III element introduced into galliumnitride (GaN).

It is reported that, in growing a semiconductor of, for example, galliumindium nitride (GaInN), nitrogen (N₂) is preferably used as a materialcarrier gas with the growth temperature for the semiconductor set toapproximately 800° C. (Applied Physics Letters, Vol. 59, pp. 2251-2253,1991).

On the other hand, it is also known that the first and second claddinglayers 304 and 308 and the first and second light guiding layer 305 and307 not including indium are preferably grown at a growth temperature of1000° C. or more with hydrogen (H₂) used as a carrier gas.

The fabrication processes for these semiconductor layers are disclosedin, for example, Japanese Laid-Open Patent Publication No. 6-196757 or6-177423.

The outline of the processes will now be described with reference toFIG. 11.

First, with hydrogen introduced onto a substrate 301, the principalplane of the substrate 301 is subjected to a heat treatment at atemperature of approximately 1050° C. Then, after lowering the substratetemperature to approximately 510° C., ammonia (NH₃) and trimethylgallium(TMG), that is, mutually reactive gases, are introduced onto thesubstrate 301, so as to grow a buffer layer 302. Thereafter, with theintroduction of TMG stopped, the substrate temperature is increased toapproximately 1030° C., and TMG and monosilane (SiH₄) are introducedonto the substrate 301 with hydrogen used as a carrier gas, therebysuccessively growing an n-type contact layer 303, a first cladding layer304 and a first light guiding layer 305, whereas trimethylaluminum (TMA)is additionally introduced as a group III material gas in growing thefirst cladding layer 304.

Next, the introduction of the material gases is stopped, the substratetemperature is lowered to approximately 800° C., and the carrier gas ischanged to nitrogen. Subsequently, trimethylindium (TMI) and TMG areintroduced onto the substrate 301 as the group III material gases,thereby growing a MQW active layer 306.

Then, the introduction of the group III material gases is stopped, thesubstrate temperature is increased to approximately 1020° C. and a groupIII material gas, that is, TMG and TMA if necessary, andcyclopentadienylmagnesium (Cp₂Mg) including a p-type dopant areintroduced onto the substrate 301, thereby successively growing a secondlight guiding layer 307, a second cladding layer 308 and a p-typecontact layer 309.

After growing the MQW active layer 306, as a protection film for theactive layer in increasing the temperature from 800° C. to 1020° C., asemiconductor layer of GaN is formed according to the description ofJapanese Laid-Open Patent Publication No. 9-186363 or a semiconductorlayer of Al_(0.2)Ga_(0.8)N is formed according to description of, forexample, Japanese Journal of Applied Physics (Vol. 35, pp. L74-L76,1996).

In general, the vapor phase epitaxial growth is conducted in anatmosphere of reduced pressure lower than the atmospheric pressure, theatmospheric pressure or increased pressure lower than approximately 1.5atm.

A technique to suppress defects from occurring on an interface between asubstrate and gallium nitride by growing gallium nitride on a substrateof sapphire by selective growth or the like is recently tried. It isreported with respect to this technique that gallium nitride with a flatface and high crystal quality can be obtained by conducting the vaporphase epitaxial growth under reduced pressure in particular.

As described so far, as a characteristic of growth of a galliumnitride-based semiconductor, different carrier gases are used in growinga layer including indium, namely, the MQW active layer 306, and layersnot including indium, such as the first cladding layer 304 and the firstlight guiding layer 305. In general, nitrogen is used for growing theformer layer and hydrogen is used for growing the latter layers.

Accordingly, in the fabrication of a semiconductor laser diode,particularly in forming a multilayer structure including doubleheterojunction layers sandwiching an active layer by the vapor phaseepitaxial growth, it is necessary to change the carrier gas before andafter forming the active layer. Also, the substrate temperature ischanged at the same time. In changing the carrier gas, the introductionof the group III material gases such as TMG is stopped, and thesubstrate is placed in an equilibrium state where no crystal grows.

However, in the aforementioned conventional method of fabricating anitride semiconductor device, the crystal face of the grownsemiconductor layer is exposed to a high temperature of approximately1000° C. and reduced pressure lower than 1 atm while the substrate isplaced in the equilibrium state where the introduction of the group IIImaterial gases is stopped. As a result, there arises a problem thatconstituent elements are released (re-evaporated) from the crystal face.

In particular, quality degradation of the first cladding layer 304 andthe first light guiding layer 305 formed below the MQW active layer 306,particularly the first cladding layer 305 including 10% of aluminum inthe aforementioned publication, leads to quality degradation of the MQWactive layer 306. This degradation results in lowering the luminousefficiency and degrading operation characteristics, for example,increasing a threshold current, of the resultant light emitting diode orsemiconductor laser diode.

Furthermore, it is recently reported in Journal of Electronic Materials(Vol. 28, No. 3, pp. 287-289, 1999) that when gallium nitride is grownunder increased pressure, an etch pit density can be reduced so as tosuppress point defects.

On the other hand, the present inventors have found the followingproblem: When a nitride semiconductor is simply grown under increasedpressure exceeding the atmospheric pressure in the above-describedequilibrium state, the concentration of material gases is so increasedthat vapor phase reactions of ammonia with trimethylaluminum andcyclopentadienylmagnesium are caused, resulting in producingintermediate reaction products through these intermediate reactions.

Accordingly, the material gases cannot be efficiently supplied onto thegrowth face of a crystal on the substrate, resulting in extremelylowering the growth rate or preventing magnesium (Mg), that is, thep-type dopant, from being introduced into the crystal.

Furthermore, when the flow rate of a carrier gas for carrying thematerial gases is increased for avoiding the production of theintermediate reaction product, the amount of gases flowing through areaction tube is so large that vortexes and convections are caused inthe air flow within the reaction tube. As a result, the crystal cannotbe grown under stable conditions.

SUMMARY OF THE INVENTION

In consideration of the aforementioned conventional problems, an objectof the invention is improving the crystal quality of a nitridesemiconductor, particularly the crystal quality of an active region andits vicinity of a semiconductor light emitting device, so as to improvethe operation characteristics such as luminous efficiency.

In order to achieve the object, according to the present invention, thegrowth ambient pressure is changed in growing nitride semiconductors inaccordance with the composition ratio of a group III element included ineach nitride semiconductor.

Specifically, in fabrication of a nitride semiconductor device,increased pressure is employed in growing a semiconductor layerincluding an element tending tore-evaporate, such as indium, and reducedpressure is employed in growing a semiconductor layer including anelement tending to generate an intermediate reaction product, such asaluminum or magnesium.

Furthermore, when epitaxial lateral overgrowth (ELO) is used, reducedpressure is employed in growing a semiconductor layer directly from aseed crystal, and the growth pressure is appropriately set in accordancewith the composition ratio of a group III element in growing anothersemiconductor layer.

Specifically, the first method of fabricating a nitride semiconductordevice of this invention comprises plural steps of respectively growingplural nitride semiconductor layers on a substrate; and between a stepof growing one nitride semiconductor layer and a step of growing anothernitride semiconductor layer adjacent to the one nitride semiconductorlayer among the plural steps, a step of changing a growth ambientpressure from a first growth ambient pressure to a second growth ambientpressure different from the first growth ambient pressure.

In the first method of fabricating a nitride semiconductor device,optimal growth ambient pressures can be set in accordance withcompositions of the plural stacked nitride semiconductor layers.Therefore, crystal dislocations can be reduced in the nitridesemiconductor layers to be grown, the nitride semiconductor layers canbe efficiently doped, and a semiconductor crystal of an active layer inparticular can be improved in the quality. As a result, the operationcharacteristics of the semiconductor device can be improved.

In the first method of fabricating a nitride semiconductor device, thefirst growth ambient pressure or the second growth ambient pressure ispreferably a pressure lower than the atmospheric pressure. In thismanner, in growing a nitride semiconductor layer including an elementtending to produce an intermediate reaction product, the intermediatereaction in a vapor phase between materials can be suppressed withoutincreasing the flow rates of material gases and carrier gases, resultingin stabilizing the growth of the crystal and improving the growthefficiency. Accordingly, the crystal quality can be improved.

In the first method of fabricating a nitride semiconductor device, amongthe plural nitride semiconductor layers, a nitride semiconductor layergrown under the pressure lower than the atmospheric pressure preferablyincludes aluminum or magnesium. In general, since a nitridesemiconductor layer including aluminum has a larger energy gap and asmaller refractive index than an active layer, such a nitridesemiconductor layer is used as a cladding layer for sandwiching theactive layer. Accordingly, the crystal quality of semiconductor layersformed in the vicinity of the active layer can be improved so as toimprove the crystal quality of the active layer in this invention, andhence, the resultant semiconductor device can attain good operationcharacteristics. Also, since a nitride semiconductor layer includingmagnesium generally exhibits a p-type conductivity, a p-typesemiconductor layer with good crystallinity can be efficiently obtained.

In the first method of fabricating a nitride semiconductor device, oneof the first growth ambient pressure and the second growth ambientpressure is preferably a pressure higher than the atmospheric pressureand the other is a pressure lower than the atmospheric pressure. In thismanner, a nitride semiconductor layer grown under the pressure lowerthan the atmospheric pressure can be improved in its growth efficiencybecause the production of an intermediate reaction product can besuppressed as described above. In addition, a nitride semiconductorlayer grown under the pressure higher than the atmospheric pressure canbe improved in its crystal quality even when it includes an elementtending to re-evaporate.

In this case, among the plural nitride semiconductor layers, a nitridesemiconductor layer grown under the pressure higher than the atmosphericpressure preferably includes indium. Since indium nitride has such ahigh vapor pressure that nitrogen can be easily re-evaporated during thegrowth, when the nitride semiconductor layer including indium is thusgrown under increased pressure, the re-evaporation of nitrogen can besuppressed.

Also in this case, the nitride semiconductor layer including indium ispreferably an active layer. In general, an active layer of a doubleheterojunction type of a light emitting device is required to have thesmallest energy gap and the largest refractive index, and hence, anitride semiconductor including indium is used as the active layer.Accordingly, the crystal quality of the active layer can be definitelyimproved in this invention.

In the first method of fabricating a nitride semiconductor device, thestep of growing the one nitride semiconductor layer and the step ofgrowing the adjacent nitride semiconductor layer are preferablyconducted at different growth temperatures. In general, a nitridesemiconductor mainly including gallium is grown at a growth temperatureexceeding 1000° C. However, when the nitride semiconductor layerincludes an element such as indium having a high vapor pressure duringthe growth, the re-evaporation of nitrogen from indium nitride can besuppressed by setting the growth temperature to a lower temperature.Thus, the crystal quality can be definitely improved.

The second method of fabricating a nitride semiconductor device of thisinvention comprises the steps of forming plural seed crystals on asubstrate: selectively growing, on the substrate, a first nitridesemiconductor layer from the plural seed crystals under a first growthambient pressure; and growing, on the first nitride semiconductor layer,a second nitride semiconductor layer under a second growth ambientpressure different from the first growth ambient pressure.

According to the second method of fabricating a nitride semiconductordevice, in the first nitride semiconductor layer grown from the pluralseed crystals in the lateral direction (the direction along thesubstrate surface), the lateral growth can be accelerated when it isgrown under the first growth ambient pressure of, for example, reducedpressure. Therefore, the nitride semiconductor layer with a flat facecan be formed over the substrate. Furthermore, when the second nitridesemiconductor layer grown on the first nitride semiconductor layer isgrown under the second ambient pressure different from the first ambientpressure, for example, under an optimal ambient pressure as in the firstmethod of fabricating a nitride semiconductor device, the nitridesemiconductor device with good quality can be formed on the firstnitride semiconductor layer including few defects.

In the second method of fabricating a nitride semiconductor device, thefirst growth ambient pressure is preferably lower than the atmosphericpressure.

In the second method of fabricating a nitride semiconductor device, afirst growth temperature employed for growing the first nitridesemiconductor layer and a second growth temperature employed for growingthe second nitride semiconductor layer are preferably different fromeach other. In this manner, the crystal growth of the first nitridesemiconductor layer and the second nitride semiconductor layer grownthereon can be individually optimized.

In this case, the second growth temperature is preferably higher thanthe first growth temperature. In this manner, the crystal orientation ofthe second nitride semiconductor layer can be further improved.

In the second method of fabricating a nitride semiconductor device, thefirst nitride semiconductor layer preferably includes aluminum. In thismanner, the surface of the first nitride semiconductor layer can berigid, so as to prevent the surface of the first nitride semiconductorlayer from degrading before starting growing the second nitridesemiconductor layer. As a result, the crystallinity of the secondnitride semiconductor layer can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a), 1(b) and 1(c) are cross-sectional views for showingprocedures in a method of fabricating a nitride semiconductor laserdiode according to Embodiment 1 of the invention;

FIG. 2 is a cross-sectional view for showing another procedure in themethod of fabricating a nitride semiconductor laser diode of Embodiment1;

FIG. 3 is a graph for showing change with time of growth pressureemployed in the method of fabricating a nitride semiconductor laserdiode of Embodiment 1;

FIG. 4 is a schematic front view of a pressure variable MOVPE system ofEmbodiment 1;

FIGS. 5(a), 5(b) and 5(c) show dependency on growth pressure of anitride semiconductor layer in the method of fabricating a nitridesemiconductor laser diode of Embodiment 1, wherein FIG. 5(a) is a graphof the dependency on growth pressure of a growth rate of aluminumgallium nitride, FIG. 5(b) is a graph of the dependency on growthpressure of a concentration of magnesium introduced into gallium nitrideand FIG. 5(c) is a graph of the dependency on growth pressure of aconcentration of magnesium introduced into aluminum gallium nitride;

FIG. 6 is a graph for showing the relationship between a growth rate ofgallium nitride and a total flow rate of material gasses and carriergases in the method of fabricating a nitride semiconductor laser diodeof Embodiment 1;

FIG. 7 is a graph for showing dependency on growth pressure of anoptimal growth temperature for a nitride semiconductor including indiumin the method of fabricating a nitride semiconductor laser diode ofEmbodiment 1;

FIG. 8 is a cross-sectional view for showing a procedure in a method offabricating a nitride semiconductor laser diode according to Embodiment2 of the invention;

FIG. 9 is a cross-sectional view for showing another procedure in themethod of fabricating a nitride semiconductor laser diode of Embodiment2;

FIG. 10 is a graph for showing change with time of a growth pressureemployed in the method of fabricating a nitride semiconductor laserdiode of Embodiment 2; and

FIG. 11 is a cross-sectional view for showing the structure of aconventional nitride semiconductor laser diode.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment 1

Embodiment 1 of the invention will now be described with reference tothe accompanying drawings.

FIGS. 1(a) through 1(c) and 2 are sectional views for showing proceduresin a method of fabricating a nitride semiconductor laser diode ofEmbodiment 1.

First, as is shown in FIG. 1(a), with a growth temperature set toapproximately 500° C., trimethylgallium (TMG) serving as a galliumsource and ammonia (NH₃) serving as a nitrogen source are introducedonto a substrate 11 of sapphire by metal organic vapor phase epitaxialgrowth (MOVPE), so as to form a buffer layer 12 of GaN for relaxinglattice mismatch between sapphire and a gallium nitride-basedsemiconductor. Then, after the substrate temperature is increased toapproximately 1020° C., TMG serving as a gallium source,trimethylaluminum (TMA) serving as an aluminum source, if necessary, NH₃serving as a nitrogen source and monosilane (SiH₄) including siliconserving as an n-type dopant are introduced onto the substrate 11, so asto successively grow, on the buffer layer 12, an n-type contact layer 13of n-type GaN, a first cladding layer 14 of n-type Al_(0.1)Ga_(0.9)N, afirst light guiding layer 15 of n-type GaN and a first protection layer16 of n-type Al_(0.2)Ga_(0.8)N. In this case, hydrogen is mainly used asa carrier gas, a growth ambient pressure (hereinafter simply referred toas the growth pressure) is set to approximately 300 Torr (approximately0.4 atm) lower than the atmospheric pressure, whereas 1 Torr isapproximately 133.322 Pa.

Next, as is shown in FIG. 1(b), the introduction of the group IIImaterial gases is stopped, the growth pressure is changed to increasedpressure of approximately 840 Torr (approximately 1.1 atm) and thegrowth temperature is lowered to approximately 780 through 800° C. Atthis point, while the introduction of the group III material gases isstopped, the crystal face of the first protection layer 16 is placed inan equilibrium state, but degradation in the face flatness can beavoided because the bonding strength between the constituent atoms ishigh in the first protection layer including aluminum (Al).

Furthermore, after changing the carrier gas from hydrogen to nitrogen.TMG, NH₃ and SiH₄ are introduced onto the substrate 11, so as to grow asecond protection layer 17 of n-type GaN on the first protection layer16. Subsequently, the introduction of SiH₄ is stopped, and a multiplequantum well (MQW) active layer 18 is grown on the second protectionlayer 17 with the ratio of introducing trimethylindium (TMI) and TMGchanged between a well layer and a barrier layer. In this case, the MQWactive layer 18 includes, for example, three cycles of the well layer ofIn_(0.09)Ga_(0.91)N with a thickness of approximately 3 nm and thebarrier layer of In_(0.01)Ga_(0.99)N with a thickness of approximately 6nm. The MQW active layer 18 may additionally include a dopant such assilicon. Also, the carrier gas may be an inert gas such as argon insteadof nitrogen.

Next, as is shown in FIG. 1(o), with keeping the increased pressure,TMG, TMA, NH₃ and Cp₂Mg serving as a p-type dopant are introduced ontothe substrate 11 while increasing the growth temperature toapproximately 1020° C., so as to grow, on the MQW active layer 18, anevaporation suppressing layer 19 of p-type AlGaN for suppressingre-evaporation of nitrogen included in the MQW active layer 18.Thereafter, the introduction of the group III material gases is stopped,and the growth pressure is set again to reduced pressure ofapproximately 400 Torr (approximately 0.53 atm). After the growthpressure and the growth temperature attain the set values, TMG, TMA ifnecessary, NH₃ and cyclopentadienylmagnesium (Cp₂Mg) serving as thep-type dopant are introduced onto the substrate 11, so as tosuccessively grown, on the evaporation suppressing layer 19, a secondlight guiding layer 20 of p-type GaN, a second cladding layer 21 ofp-type Al_(0.1)Ga_(0.9)N and a p-type contact layer 22 of p-type GaN.

The evaporation suppressing layer 19 is grown with the growth pressureset to the increased pressure as in growing the MQW active layer 18while increasing the growth temperature for the purpose of preventingindium nitride (InN) included in the MQW active layer 18 from degradingin its crystal quality through decomposition during the temperatureincrease. When the growth rate is sufficiently small, for example,approximately 1 nm/min., the evaporation suppressing layer 19 cansufficiently exhibit its function against the MQW active layer 18. Inaddition, when the evaporation suppressing layer 19 sufficiently coversthe MQW active layer 18, the MQW active layer 18 can be free from damageeven if the growth pressure is changed from the increased pressure tothe reduced pressure and the carrier gas is changed from nitrogen tohydrogen.

Next, as is shown in FIG. 2, the second cladding layer 21 and the p-typecontact layer 22 are etched so as to form a ridge 30 with a width ofapproximately 5 μm. Subsequently, an insulating film 23 of silicon oxide(SiO₂) is formed over the second light guiding layer 20 including thetop and side faces of the ridge 30 by CVD or the like. Then, the n-typecontact layer 13 is etched so as to expose a portion on a side of theridge 30.

Subsequently, an opening is formed in the insulating film 23 on theridge 30, and a p-side electrode 24 including stacked layers of, forexample, nickel (Ni) and gold (Au) is formed by evaporation or the likeso as to cover the ridge 30 and be in ohmic contact with the p-typecontact layer 22 through the opening.

Furthermore, an n-side electrode 25 including stacked layers of, forexample, titanium (Ti) and aluminum (Al) is formed by the evaporation orthe like on the exposed portion of the n-type contact layer 13.

In the nitride semiconductor laser diode thus fabricated, when apredetermined voltage is applied to the p-side electrode 24 with then-side electrode 25 grounded, holes and electrons are injectedrespectively from the p-side electrode 24 and the n-side electrode 25into the MQW active layer 18, so as to generate optical gain within theMQW active layer 18, resulting in showing laser action at a wavelengthof approximately 405 nm.

In this embodiment, the crystal quality of the MQW active layer 18 issuppressed from degrading by forming the first protection layer 16 ofn-type Al_(0.2)Ga_(0.8)N and the second protection layer 17 of n-typeGaN below the MQW active layer 18 and forming the evaporationsuppressing layer 19 of p-type AlGaN on the MQW active layer 18, butthese protection layers and the like are not indispensable members ofthe laser diode.

Furthermore, although the substrate 11 is made from sapphire, thesubstrate may be made from silicon carbide (SiC) or silicon (Si) insteadof sapphire or may be a silicon substrate whose upper portion iscarbonated.

FIG. 3 shows change with time of the growth pressure in the method offabricating a nitride semiconductor laser diode of this embodiment.

As is shown in FIG. 3, in first growth pressure time t1 in which thegrowth pressure is approximately 300 Torr, the buffer layer 12, then-type contact layer 13, the first cladding layer 14, the first lightguiding layer 15 and the first protection layer 16 are successivelygrown. Then, in second growth pressure time t2 in which the growthpressure is approximately 840 Torr, the second protection layer 17, theMQW active layer 18 and the evaporation suppressing layer 19 aresuccessively grown. Subsequently, in third growth pressure time t3 inwhich the growth pressure is approximately 400 Torr, the second lightguiding layer 20, the second cladding layer 21 and the p-type contactlayer 22 are successively grown.

Now, a pressure variable MOVPE system capable of changing the growthpressure as is shown in FIG. 3 will be described with reference to theaccompanying drawing.

FIG. 4 schematically shows the pressure variable MOVPE system accordingto this embodiment. As is shown in FIG. 4, the pressure variable MOVPEsystem 100 includes a reaction chamber 101; an inlet tube 102 ofstainless steel or quartz for introducing material gases and carriergases for carrying the material gases into the reaction chamber 101; arotary pump 103 disposed on the exhausting side of the inlet tube 102for exhausting unnecessary material gases and the like from the reactionchamber 101; and a conductance valve 105 disposed between the reactionchamber 101 and the rotary pump 103, which is opened or closed inaccordance with a pressure gauge 104 for measuring the pressure withinthe reaction chamber 101 so as to adjust the pressure within thereaction chamber 101 to the atmospheric pressure, reduced pressure lowerthan the atmospheric pressure or increased pressure up to several atm.

Partitions are provided within the inlet tube 102 so that a group IIImaterial source, a group V material source and a sub-flow gas can beindependently introduced to the vicinity of a substrate. The sub-flowgas is nitrogen, hydrogen or an inert gas such as argon, and isintroduced in parallel with material gases for suppressing the materialgasses from blowing up above the substrate due to a convection or thelike.

The inlet tube 102 is provided with a window on its side facing thebottom of the reaction chamber 101. A susceptor 106 for supporting asubstrate 110 on its lower and side faces is fit in the window, and thetop face of the susceptor 106 is placed at the same level as the innerwall of the inlet tube 102.

A heater 107 for heating the substrate 110 is disposed below thesusceptor 106 within the reaction chamber 101. The heater 107 isexternally monitored through, for example, a thermoelectric couple 108for adjusting the temperature of the substrate 110 to a desiredtemperature.

Now, the characteristic and the effectiveness of variable MOVPE wherethe growth pressure is changed during the growth of nitridesemiconductors will be described with reference to FIGS. 5(a) through5(c).

FIG. 5(a) shows dependency on the growth pressure of a growth rate ofaluminum gallium nitride (AlGaN). As is shown in FIG. 5(a), as thegrowth pressure is higher, the growth rate is lower. In particular, whenthe growth pressure exceeds approximately 1 atm, the growth rate isexcessively lowered.

FIG. 5(b) shows dependency on the growth pressure of a concentration ofmagnesium (Mg) introduced into gallium nitride (GaN). As is shown inFIG. 5(b), the concentration of Mg introduced into p-type GaN is morelargely lowered as the growth pressure increases. In particular, whenthe growth pressure exceeds approximately 1 atm, the concentration of Mgis excessively lowered.

This is probably because the probability of collision between thematerials is increased in a vapor phase when the growth pressure ishigh, and in particular, an intermediate reaction is caused betweentrimethylaluminum (TMA) and ammonia (NH₃) or betweencyclopentadienylmagnesium (Cp₂Mg) and ammonia (NH₃), so that thematerials cannot be efficiently supplied onto the substrate.

In proof of this, the concentration of magnesium (Mg) introduced intoaluminum gallium nitride (AlGaN) shown in FIG. 5(c) is more largelylowered as the growth pressure increases than that introduced intogallium nitride shown in FIG. 5(b).

Accordingly, a semiconductor layer of p-type AlGaN is very efficientlygrown under reduced pressure lower than the atmospheric pressure.

On the other hand, a gallium nitride-based semiconductor includingindium is efficiently grown at a low temperature or under high growthpressure for suppressing re-evaporation of nitrogen because indiumnitride (InN) has such a high vapor pressure that it is necessary tosuppress defects due to release of nitrogen. Accordingly, a galliumnitride-based semiconductor including indium is conventionally generallygrown by atmospheric MOVPE conducted under the atmospheric pressure andis sometimes grown by increased pressure MOVPE.

In the conventional atmospheric MOVPE and the increased pressure MOVPE,however, the growth pressure is always set to a fixed value.Accordingly, there has been no disclosure of a growth method in whichthe growth pressure is changed, specifically, increased pressure isemployed in growing the MQW active layer 18 of a gallium nitride-basedsemiconductor including In for attaining high crystal quality andreduced pressure is subsequently employed in growing the second lightguiding layer 20, the second cladding layer 21 and the like of galliumnitride-based semiconductors including Al or Mg for suppressing theproduction of intermediate reaction products in the vapor phase as inthis embodiment.

Now, in order to suppress the production of intermediate reactionproducts, a method for reducing the probability of collision between thematerials by lowering the concentrations of the material gases insteadof employing reduced pressure as in this embodiment will be verified.

FIG. 6 shows the relationship between a growth rate of gallium nitrideand a total flow rate of material gases and a carrier gas. In FIG. 6,the abscissa indicates the number of times of growth and the ordinateindicates the growth rate of gallium nitride. Also, the growth pressureis set to approximately 840 Torr (approximately 1.1 atm) and the totalflow rate is adjusted by increasing/decreasing the flow rate of hydrogenor nitrogen serving as a carrier gas for a group III material.

In the case shown with a solid line 1 in FIG. 6 where the total flowrate is 20 through 40 slm (standard liter per minute) and a vapor phaseintermediate reaction occurs, the growth rate of gallium nitride issubstantially constant and stable through repeated growth. In contrast,in the case shown with a broken line 2 where the total flow rate exceeds40 slm, the flow rate is so high that the growth rate is lowered becausethe efficiency of thermal decomposition of the materials is degraded orthat the air flow is unstably changed due to vortexes or a small amountof reaction product generated therein. As a result, the growth ratebecomes more and more unstable through repeated growth.

Therefore, when the pressure variable MOVPE is employed, an active layerof a nitride semiconductor including indium is grown under increasedpressure so as to reduce defects resulting from holes from whichnitrogen has been released, and a nitride semiconductor not includingindium is grown under reduced pressure so as to suppress the vapor phaseintermediate reaction. As a result, crystals can be stably and highlyefficiently grown.

Furthermore, the present inventors have found that when a nitridesemiconductor including indium is grown under increased pressure, thegrowth temperature can be higher than the case where the increasedpressure is not employed.

FIG. 7 shows dependency on growth pressure of an optimal growthtemperature of a nitride semiconductor including indium, wherein theabscissa indicates the growth pressure and the ordinate indicates thegrowth temperature. In FIG. 7, a solid line 3 shows the dependencyobtained in a MQW layer including three cycles of InGaN including indiumin a composition ratio smaller than 7% and GaN, and a broken line 4shows the dependency obtained in a GaN layer.

As is shown in FIG. 7, when the growth pressure is 840 Torr(approximately 1.1 atm), the optimal growth temperature of the MQW layer3 is approximately 800° C., which is lower than that of the GaN layer 4by approximately 190° C. Furthermore, when the growth pressure isincreased to 1200 Torr (approximately 1.6 atm), the optimal growthtemperature of the MQW layer 3 is approximately 830° C., which is lowerthan that of the GaN layer 4 by approximately 160° C. On the other hand,when the growth pressure is reduced to 300 Torr (approximately 0.4 atm),the optimal growth temperature of the MQW layer 3 is approximately 755°C., which is lower than that of the GaN layer 4 by approximately 215° C.In this manner, the optimal growth temperature of the MQW layer 3including indium is higher when it is grown under increased pressurethan when it is grown under reduced pressure.

Therefore, when an active layer including indium is grown underincreased pressure, it can be grown at a higher temperature, so as tofurther improve the crystallinity of the active layer. In addition, atemperature difference from the growth temperature of p-type GaN andp-type AlGaN, of approximately 1000° C., to be grown on the active layercan be made small, and hence, damage of the active layer due to thetemperature difference can be reduced in increasing the temperature tothe growth temperature of the semiconductor layers to be grown on theactive layer.

The semiconductor laser diode of FIG. 2 thus fabricated by the pressurevariable growth method of this embodiment has a threshold currentapproximately ½ as small as that of a conventional laser diode.

Although the active layer is grown under increased pressure higher thanthe atmospheric pressure and the semiconductor layers other than theactive layer are grown under reduced pressure in this embodiment, thesemiconductor layers other than a p-type GaN layer and a p-type AlGaNlayer, where it is necessary to suppress the production of theintermediate reaction products, may be grown under pressure equivalentto or higher than the atmospheric pressure. This is because the vaporphase intermediate reaction is substantially negligible in the growth ofa semiconductor layer not including aluminum and magnesium. Furthermore,in changing the growth pressure, there is no need to always stop thegrowth but a subsequent layer may be continuously grown by lowering thegrowth rate by, for example, reducing the supply amount of the group IIImaterial.

Furthermore, although the method of fabricating a gallium nitride-basedsemiconductor laser diode is described in this embodiment, the method isalso very effective in growing an active region of a light emittingdiode device or an electronic device. For example, when this method isemployed, the luminous efficiency can be largely improved in a lightemitting diode device, and the mobility of carriers in an active layercan be increased in an electronic device.

Embodiment 2

Embodiment 2 of the invention will now be described with reference tothe accompanying drawings.

In Embodiment 1, in order to improve the crystal quality of an activelayer and reduce products generated through the vapor phase intermediatereaction between material gases, the growth pressure is set to differentvalues between the growth of the active layer and the growth ofsemiconductor layers sandwiching the active layer and having a smallerrefractive index than the active layer. In Embodiment 2, another methodfor improving the quality of the active layer will be described.

FIGS. 8 and 9 are sectional views for showing procedures in a method offabricating a nitride semiconductor laser diode of Embodiment 2.

First, as is shown in FIG. 8, seed crystal layers 42 of GaN in the shapeof plural ridge stripes extending at intervals are formed on a substrateof, for example, sapphire. The seed crystal layers 42 can be formed, forexample, by growing a semiconductor layer of GaN in a predeterminedthickness on the substrate by the MOVPE or the like and patterning thesemiconductor layer by known photolithography and etching.Alternatively, the seed crystal layers 42 may be formed by depositing,on the substrate 41, a mask pattern of a dielectric film of siliconoxide or the like having plural stripe-shaped openings and conductingepitaxial lateral overgrowth (ELO) by using the mask pattern.

Then, with the growth pressure set to approximately 100 Torr(approximately 0.13 atm) and with the substrate temperature set toapproximately 950° C., material gases, namely, trimethylgallium (TMG),trimethylaluminum (TMA), ammonia (NH₃) and monosilane (SiH₄) includingan n-type dopant are introduced onto the substrate 41 by the MOVPE.Thus, a flattening layer 43 of n-type AlGaN is grown on the substrate 41through the ELO from the side faces of the seed crystal layers 42 so asto be integrated and have a flat top face.

Next, with the growth pressure set to approximately 300 Torr(approximately 0.4 atm) and with the substrate temperature set toapproximately 1050° C., TMG, TMA, NH₃ and SiH₄ are introduced onto thesubstrate 41, so as to grow a first cladding layer 44 of n-type AlGaN onthe flattening layer 43. A line 43 a vertical to the substrate surfaceformed in the flattening layer 43 above each seed crystal layer 42 orbetween the seed crystal layers 42 corresponds to a junction wherecrystals respectively grown from the seed crystal layers 42 meet eachother before forming the integrated flattening layer 43.

Also at this point, the present inventors have found the following: Whenthe flattening layer 43 of n-type AlGaN is grown at a low growthtemperature so that the materials can be easily deposited in thevicinity of the side faces of the respective seed crystal layers 42 onthe substrate 41 with the collision of the materials suppressed bysetting the growth pressure to reduced pressure of 100 Torr so that theflattening layer 43 can be uniformly grown from the side faces of theseed crystal layers 42 in a lateral direction (a direction parallel tothe substrate surface), the quality of a semiconductor crystalsubsequently grown on the flattening layer 43 can be effectivelyimproved.

Furthermore, it has been found that since the flattening layer 43 grownat a low temperature is poor in the crystallinity such as the C-axisorientation, when the first cladding layer 44 is grown at a growthtemperature higher than that for the flattening layer 43 byapproximately 100° C., the crystallinity of the first cladding layer 44can be improved.

Although the flattening layer 43 is made from n-type AlGaN in thisembodiment, it may not include aluminum or may not be of n-type.However, the flattening layer 43 preferably includes aluminum becausesurface degradation of the flattening layer 43 can be thus avoided inincreasing the growth pressure from 100 Torr to 300 Torr for growing thefirst cladding layer 44.

The surface of the thus obtained first cladding layer 44 is etched byusing an etchant including phosphoric acid and sulfuric acid and theetch pit density of the etched surface is observed. Thus, it isconfirmed that the etch pit density is reduced by approximately twofigures as compared with that in n-type AlGaN obtained by a conventionalmethod.

Although the substrate 41 is made from sapphire in this embodiment, thesubstrate may be made from silicon carbide (SiC) or silicon (Si) insteadof sapphire or may be a silicon substrate whose upper portion iscarbonated.

The seed crystal layer 42 is not limited to GaN but may include Al or Inbecause it can serve as a seed crystal of AlGaN even when it includes Alor In.

Next, as is shown in FIG. 9, respective nitride semiconductor layersconstructing the laser diode are grown in the same manner as inEmbodiment 1.

Specifically, with the substrate temperature set to approximately 1050°C. and the growth pressure set to approximately 300 Torr (approximately0.4 atm), TMG, NH₃ and SiH₄ are introduced onto the substrate 41, so asto grow a first light guiding layer 45 of n-type GaN on the firstcladding layer 44.

Subsequently, with the introduction of the group III material gasesstopped, the growth pressure set to increased pressure of approximately840 Torr (approximately 1.1 atm), the growth temperature lowered toapproximately 780 through 800° C. and the carrier gas changed fromhydrogen to nitrogen, TMI, TMG and NH₃ are introduced onto the substrate41, so as to grow, on the first light guiding layer 45, a MQW activelayer 46 including stacked InGaN layers respectively having differentcomposition ratios of In.

While keeping the increased pressure and increasing the growthtemperature to approximately 1020 through 1050° C., TMG, TMA, NH₃ andCp₂Mg are introduced onto the substrate 41, so as to grow an evaporationsuppressing layer 47 of p-type AlGaN on the MQW active layer 46.Thereafter, with the introduction of the group III material gasesstopped and the growth pressure set again to reduced pressure ofapproximately 400 Torr (approximately 0.53 atm), TMG, TMA if necessary,NH₃ and Cp₂Mg are introduced onto the substrate 41, so as tosuccessively grow, on the evaporation suppressing layer 47, a secondlight guiding layer 48 of p-type GaN, a second cladding layer 49 ofp-type AlGaN and a p-type contact layer 50 of p-type GaN.

Thereafter, an upper portion of the second cladding layer 49 and thep-type contact layer 50 are formed into a ridge 30, and an insulatingfilm 51 is formed so as to cover the remaining portion of the secondcladding layer 49 including the ridge 30. At this point, since a crystalabove every junction 43 a of the flattening layer 43 has a highdislocation density, the ridge 30 is preferably formed in a region notincluding a portion above the junction 43 a.

Subsequently, a p-side electrode 52 is formed so as to cover the ridge30, and an n-side electrode 53 is formed on an exposed portion of thefirst cladding layer 44.

In the semiconductor laser diode of this embodiment, the thresholdcurrent can be reduced to approximately ½ of that of a laser diodefabricated by a conventional method. Furthermore, the present laserdiode can be continuously operated at room temperature for more than10000 hours, and it is thus confirmed that the life can be remarkablyimproved.

FIG. 10 shows change with time of the growth pressure in the method offabricating a nitride semiconductor laser diode of this embodiment.

As is shown in FIG. 10, in first growth pressure time t1 in which thegrowth pressure is approximately 100 Torr, the flattening layer 43 isgrown. Next, in second growth pressure time t2 in which the growthpressure is approximately 300 Torr, the second cladding layer 44 and thefirst light guiding layer 45 are successively grown. Subsequently, inthird growth pressure time t3 in which the growth pressure isapproximately 840 Torr, the MQW active layer 46 and the evaporationsuppressing layer 47 are successively grown. Then, in fourth growthpressure time t4 in which the growth pressure is approximately 400 Torr,the second light guiding layer 48, the second cladding layer 49 and then-type contact layer 50 are successively grown.

As described above, according to this embodiment, very low pressure of100 Torr is employed in growing the flattening layer 43 from the seedcrystal layers 42 through the ELO, reduced pressure of 300 Torr isemployed in growing the first cladding layer 44 on the flattening layer43, and the growth temperature is higher in growing the first claddinglayer 44 by approximately 100° C. than in growing the flattening layer43. Accordingly, even though the ELO is employed, the crystallinity ofthe MQW active layer 46 grown on the first cladding layer 44 can befurther improved.

In addition, similarly to Embodiment 1, the growth pressure is set toincreased pressure in growing a semiconductor layer where re-evaporationis easily caused, and is set to reduced pressure in growing asemiconductor layer where an intermediate reaction product is easilyproduced. Since optimal growth pressures are thus selected, it ispossible to form the MQW active layer 46 of InGaN with high quality onthe first cladding layer 44 of n-type AlGaN having a low defect density.

Accordingly, in the growth of a semiconductor layer including aluminumor magnesium where an intermediate reaction product is easily produced,the vapor phase intermediate reaction can be suppressed, so as toefficiently and stably grow a semiconductor of p-type AlGaN or p-typeGaN.

Also in this embodiment, at least one of a first protection layer ofn-type AlGaN and a second protection layer of n-type GaN can be formedbetween the first light guiding layer 45 and the MQW active layer 46.

Moreover, although the active layer is grown under increased pressurehigher than the atmospheric pressure and the semiconductor layers otherthan the active layer are grown under reduced pressure in thisembodiment, semiconductor layers other than a p-type GaN layer and ap-type AlGaN layer where it is necessary to suppress the production ofthe intermediate reaction products may be grown under a pressureequivalent to or higher than the atmospheric pressure.

Furthermore, although the method of fabricating a gallium nitride-basedsemiconductor laser diode is described in this embodiment, the method isalso very effective in growing an active region of a light emittingdiode device or an electronic device. For example, when this method isemployed, the luminous efficiency can be largely improved in a lightemitting diode device, and the mobility of carriers in an active layercan be increased in an electronic device.

The method of fabricating a nitride semiconductor of Embodiment 1 or 2is applicable not only to the MOVPE but also to any method capable ofgrowing a nitride semiconductor layer such as hydride vapor phaseepitaxial growth (HVPE) and molecular beam epitaxial growth (MBE).

1. A method of fabricating a nitride semiconductor device by a vapordeposition method comprising the steps of: forming plural seed crystalson a substrate; selectively growing, on said substrate, a first nitridesemiconductor layer including aluminum from said plural seed crystalsunder a first growth ambient pressure; and growing, on said firstnitride semiconductor layer, a second nitride semiconductor layer undera second growth ambient pressure different from said first growthambient pressure, wherein said first growth ambient pressure is lowerthan said second growth ambient pressure, and is lower than theatmospheric pressure.
 2. The method of fabricating a nitridesemiconductor device of claim 1, wherein a first growth temperatureemployed for growing said first nitride semiconductor layer and a secondgrowth temperature employed for growing said second nitridesemiconductor layer are different from each other.
 3. The method offabricating a nitride semiconductor device of claim 2, wherein saidsecond growth temperature is higher than said first growth temperature.